Advances in data storage capability have been manifested as faster data scanning times as well as the density of information which can be stored on the data record carrier. The desire for higher areal data densities has led to the development of a number of innovative data storage techniques.
Optical scanning devices are well known in the art, and include rewritable record carrier systems, like the CD-RW system. In these systems, data is written to and read from an optically varying medium by optical modulation. Increasing the data density by decreasing the dimensions of the data bits (usually pits in the optical surface of the record carrier) requires accurate imaging of the bits. A reduction in the bit size requires short wavelength or near-field operation and it is considered that present technologies are approaching their limit in terms of the signal to noise (SNR) requirements while still maintaining a useful system bandwidth.
Magnetic scanning devices, such as those used in hard disk drives, in which data is written to and read from a magnetisable medium by magnetic modulation, can exhibit high areal data densities by employing relatively short bit lengths. However, those systems use larger track widths than optical scanning devices, which imposes a significant limitation on the data density which may be recorded on a media. In this context, higher areal densities may also be possible by reducing the bit lengths even further. However, the same SNR will be required for the same bit error rate. In magnetic media, smaller bits require the use of even smaller particles with about 100 particles per bit being required for a reasonable SNR.
Smaller bit sizes become thermally unstable when used with media coercivities which are still writeable at approximately room temperature. This limitation can be overcome by using thermally assisted writing processes with high coercivity media. According to this technique, media with a coercivity which is too high to be written with current writing head technology at room temperature, is heated thereby lowering the coercivity. The media may then be written using magnetic modulation provided by current technology recording heads.
Magneto-optical scanning devices are also known. In those systems, data is written to a magnetisable medium by optical and magnetic means. Optical and/or magnetic modulation may be employed. However, in all such systems, the area data densities are defined by the optical part of the system, since the bit sizes are defined by the optical spot size. Reading from the medium is conducted optically.
Other hybrid systems, which combine aspects of both magnetic and optical technologies to push data storage densities to higher levels, have been proposed in: “H. Saga, H. Nemoto, H. Sukeda and M. Takahasji, ‘New recording method combining thermo-magnetic writing and flux detection’, Japanese Journal of Applied Physics, Pt 1, Vol 38, No 3B, March 1999, pp 1839-40”; and H. Katayama, S. Sawamura, Y. Ogimoto, J. Nakajima, K. Kojima and K. Ohta, ‘New magnetic recording method using laser assisted read/write technologies’, J. Magn. Soc. Japan, Vol. 23, Supplement No. S1, 1999, pp. 233-236.
JP-A-4-311848 describes optical track following in conjunction with magnetic reading and writing. The magnetic scanning head is located on the opposite side of the record carrier to the optical tracking head. The position of the scanning head is determined by means of magnetic field detection coils associated with the optical tracking head. These coils detect the modulated magnetic writing field thus allowing the position of the scanning head to be determined. However, the positioning accuracy is not as accurate as that exhibited by optical tracking systems.
JP-A-2-105319 describes a ‘side-by-side’ system whereby tracks defined by wobble bits formed on the on the record carrier, which are tracked using a laser, are placed next to magnetic data tracks. The area taken up by the wobble bits reduces the media area available for magnetic reading and writing.